config: config file path from STARCLUSTER_CONFIG env. variable

Modified `StarClusterConfig.__init__()` to get the config file path from the `STARCLUSTER_CONFIG` environment variable. Useful in my case because I have separate AWS accounts/dev. environments for work and personal projects, and I already switch them by sourcing a file that changes shell environment variables. It would be nice to also change the StarCluster config file without having to supply "`-c <path>`" to every command or having to play symlink tricks with `~/.starcluster/config`.